AstarothMastemaRavenclaw wrote:

I currently am happy running Arch, but I would like to build a source-based distro (Gentoo, Source Mage), or even LFS if only for the great learning experience involved (as a summer project/hobby) . But as these take a very long time to build, I was wondering if it's possible to build it on a partition on my terrabyte drive, and then when I am done, export an installable .iso of the system for my acutal computer(s) ?

That's totally doable. In fact, that was the gentoo old way. I remember running a livecd, chroot to the hard drive and start compiling from Stage 1!!!!! that way of installation took around 2 days to get a basic system hehehe good old days when I had time...

Any linux installation is "moveable" to another hard drive, you'll just have to set the proper configs on the fstab and the boot loader. That's all you need :) (and the support for your hardware on the kernel, obviously)

Regards

You can easily do it with Gentoo, that's currently what you do when you install it in a drive. No matter if you don't finish the installation the same day and you want to contine with it later on.
Now a Stage3 install is preferred but as Marc mentioned you can even install from Stage1 and it could take a lot of hours (or days...). My current record is a 4 days installation (24h x 4d without reboots) for a Sparc machine (SUN Sparc Station 5), in that case for a bare bone install I was forced to start with Stage1 because that's what you normally do with sparc architectures (nobody provided stage3 images for sparc). This means installing GLIBC, GCC, SILO (LILO for sparc), bash and few more: emerge, emerge, emerge, emerge....
Now with a common PC you may install a basic gentoo system in a couple of hours with USE flags and proper optimizations as well.
With the same approach you may even install gentoo in your hard drive for a different machine and using a different architecture (ARM for example), cross compilation and toolchains are really easy to use on it.